Modern vehicle engines have been called upon to supply energy for an increasing number of accessories, some of which are for passenger comfort and convenience, and some of which are for vehicle operation and safety, as for example, air conditioning, alternators, engine fans, power steering pumps and power brake devices. Generally each accessory is energized from a component which has an optimum rotational speed; ideally, each component should be driven at that optimum speed. This is not practical because of the relatively high costs of such drives, and the amount of space which would be necessary for such drives. If an accessory drive system is constructed to drive accessories at speeds which are directly proportional to either engine speed or vehicle ground speed, the accessories must be constructed to withstand maximum running speeds and, in all probability, the accessories will not be operated at their optimum conditions. Energy is wasted when the components are driven at a speed above the optimum value. Thus it has been proposed to construct an accessory drive system in which the driven speed of the accessory power system or energy translator generally increases at a rapid rate until the driven speed reaches a certain, predetermined desirable speed at engine idle, after which the accessory supplies are driven at a substantially constant speed. This constant speed, under ideal circumstances, closely matches the optimum drive speed. By doing so, the accessory drive system can be constructed for optimum performance and life, and at reasonable cost. Also, fuel is not wasted driving the accessory power supplies at excessive speeds. Some accessory drive packages use a pair of pulleys in a "variable pulley" system. An example of such an accessory drive system is described and claimed in Woolard U.S. Pat. No. 4,100,818, issued July 18, 1978. Woolard's pulley arrangement utilizes centrifugal force to vary the drive ratio between the pulleys of the pulley arrangement. The centrifugal force acting on a plurality of weights causes a pulley flange to move axially relative to the other flange of the pulley. The upper limit of the ratio change is determined by the limit of movement of the weights.
In the Woolard system, the drive and driven pulleys rotate at a fixed ratio at relatively low motor speeds up to a predetermined speed at which time the weights attached to the spring of the driven pulley begin to move outwardly. As they move, there is a change in axial spacing between the pulley flanges of each pulley. The drive pulley continues to rotate at the same speed as the drive shaft while the driven pulley (to which the accessory power supply is attached) rotates at a substantially uniform speed. This condition prevails until the drive shaft speed becomes very high at which time the weights have reached their outermost limit and the spring has "bottomed out". Thereafter the speed of the driven pulley increases with further increases in the drive shaft speed but at a reduced lower ratio as compared to the first drive ratio.
Another accessory drive system is described and claimed in U.S. Pat. No. 3,893,343 issued July 8, 1975. The drive arrangement comprises a variable "V-belt transmission" which provides a speed-up at low rotational speeds of the crankshaft and which automatically reduces the speed-up ratio with increasing rotational speeds of the crankshaft by means of a piston which is associated with the driving pulley of the transmission and which displaces the movable pulley member against spring action. The piston is acted upon directly by unmodulated lubricating oil pressure from the internal combustion engine oil pump together with the pressure developed by the oil contained in a large diameter driving pulley cylinder as it is rotated at crankshaft speeds. Accessory speeds are increased at idle but the large diameter piston forces available at speeds above idle quickly changes the belt ratio, and prevents efficient use of the transmission.